Polymer, energy absorber rail extension, methods of making and vehicles using the same

ABSTRACT

In one embodiment, a rail extension comprises: an energy absorber comprising a polymer body, and vehicle attachment tabs extending from one end of the energy absorber and configured to attach to a vehicle rail, and an attachment tab extending from another end of the energy absorber and configured to attach to a bumper beam. The energy absorber can comprise cells formed by cell walls extending a length of the energy absorber and forming cavities therethrough; and open channels formed on each side of the energy absorber, wherein the channels are defined by walls of adjacent cells. A vehicle can comprise: a bumper beam; a vehicle rail; and the rail extensions.

BACKGROUND

The present disclosure relates generally to a polymer energy absorbingrail extension.

Bumper systems generally extend widthwise, or transverse, across thefront and rear of a vehicle and are mounted to rails that extend in alengthwise direction. Many bumper assemblies for an automotive vehicleinclude a bumper beam and an injection molded energy absorber secured tothe bumper beam with a fascia covering the energy absorber.

Beneficial energy absorbing bumper systems achieve high efficiency bybuilding load quickly to just under the load limit of the rails andmaintain that load constant until the impact energy has been dissipated.Energy absorbing systems attempt to reduce vehicle damage as a result ofa collision by managing impact energy absorption. Bumper system impactrequirements are set forth by United States Federal Motor Vehicle SafetyStandards (US FMVSS), Canadian Motor Vehicle Safety Standards (CMVSS),European EC E42 consumer legislation, EuroNCAP pedestrian protectionrequirements, Allianz impact requirements, and Asian PedestrianProtection for lower and upper legs. In addition, the InsuranceInstitute for Highway Safety (IIHS) has developed different barrier testprotocols on both front and rear bumper systems. These requirements mustbe met for the various design criteria set forth for each of the variousautomotive platforms and car models. If there is even very limiteddamage to any component of the frame of the vehicle, costs of repairingthe vehicle can escalate dramatically.

This generates the need to develop low cost, lightweight, and highperformance energy absorbing systems that will deform and absorb impactenergy to ensure a good vehicle safety rating and reduce vehicle damagein low speed collisions. Different components due to their inherentgeometry and assembly requirements need different energy absorberdesigns to satisfy the impact criteria. Therefore, the automotiveindustry is continually seeking economic solutions to improve theoverall safety rating of a vehicle. Hence, there is a continual need toprovide a solution that would reduce vehicle damage and/or enhance avehicle safety rating.

BRIEF DESCRIPTION

Disclosed, in various embodiments, are energy absorbing rail extensions,methods for making and vehicles using the same.

In one embodiment, a rail extension, comprises: an energy absorbercomprising a polymer body, and vehicle attachment tabs extending fromone end of the energy absorber and configured to attach to a vehiclerail, and an attachment tab extending from another end of the energyabsorber and configured to attach to a bumper beam. The energy absorbercan comprise cells formed by cell walls extending a length of the energyabsorber and forming cavities therethrough; and open channels formed oneach side of the energy absorber, wherein the channels are defined bywalls of adjacent cells.

In one embodiment, a vehicle can comprise: a bumper beam; a vehiclerail; a rail extension; and vehicle attachment tabs extending from arail end of the rail extension and attached to the vehicle rail, and abumper attachment tab extending from another end of the rail extensionand attached to the bumper beam. The rail extension can comprise: anenergy absorber comprising a polymer body, wherein the energy absorbercomprises cells formed by cell walls extending a length of the energyabsorber and forming cavities therethrough; and open channels formed oneach side of the energy absorber, wherein the channels are defined bywalls of adjacent cells.

In one embodiment, a method of controlling a crushing of a vehicleextension, can comprise: forming a rail extension, the rail extensioncomprising an energy absorber comprising a polymer body, wherein theenergy absorber comprises cells formed by cell walls extending a lengthof the energy absorber and forming cavities therethrough; forming openchannels along the outside of the rail extension; determining an initialforce peak during crushing of the energy absorber; beveling a surface ofsome cells based upon the initial force peak; changing the angle of thebevel until the initial force peak is less than a desired maximum force.

These and other non-limiting characteristics are more particularlydescribed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings wherein likeelements are numbered alike and which are presented for the purposes ofillustrating the exemplary embodiments disclosed herein and not for thepurposes of limiting the same.

FIG. 1 is a perspective, partial view of an embodiment of abody-in-white (BIW) with polymer rail extensions located between thebumper beam and the rails.

FIG. 2 is a perspective front view of an embodiment of a polymer railextension with chamfered cells and vehicle attachments.

FIG. 3 is a perspective front view of another embodiment of a polymerrail extension without chamfered cells and with vehicle attachments.

FIG. 4 is a perspective front view of yet another embodiment of apolymer rail extension with chamfered cells and vehicle attachments.

FIG. 5 is graphical representation of rail extension performance showingdisplacement (mm) versus force (kiloNewtons (kN)) for a simulated and atest polymer rail extensions.

FIG. 6 is graphical representation of rail extension performance showingdisplacement (mm) versus force (kN)) for the simulated polymer railextension of FIG. 5 versus a metal rail extension.

DETAILED DESCRIPTION

Disclosed herein, in various embodiments, are energy absorbing deviceswhich can be used in conjunction with vehicle components, e.g., tominimize the damage suffered during an impact. Although it is envisionedthat the energy absorbing rail extensions can comprise metal inserts(e.g., strategically located metal reinforcements), these elements canbe wholly polymer elements (besides attachment inserts that can belocated in tabs configured to be attached to the vehicle). The energyabsorption section of the extensions are desirably configured to, duringimpact, maintain a substantially constant force (e.g., will vary by lessthan or equal to 20%). In other words, if the desired constant force is100 kN, the variation will not exceed 80 kN to 120 kN. It is also noted,that, desirably, during an impact, the energy absorption section impartsa force that exceeds the constant force (e.g., the maximum desiredforce) by less than or equal to 20%, specifically, less than or equal to10%, and more specifically, less than or equal to 5%. In other words, ifthe desired constant force is 100 kN, desirably, during an impact, theenergy absorption section imparts a force that is less than or equal to120 kN, specifically, less than or equal to 110 kN, and morespecifically, less than or equal to 105 kN. It is understood that theforces exerted by the energy absorption section are exerted during animpact sufficient to crush the energy absorption section, until theenergy absorption section is crushed.

In addition to maintaining a substantially constant force duringcrushing, the rail extension desirably crushes completely and does notexceed a force during crushing over the force limit for the vehicle. Theminimum force on the rail extensions that will initiate crushing isdependent upon the strength of the rails. Generally, the minimum forceto initiate crushing is greater than or equal to 60 kN, specifically,greater than or equal to 70 kN, and more specifically, greater than orequal to 80 kN. In other words, the force during impact is maintainedbelow the force limit of the rails so that the rails do not fail ordeform before the rail extensions fully crush.

The rail extensions can have multiple cells and can be alveolarstructures more commonly referred to as “honeycomb”. The combs of thestructure can be any polygonal or rounded shape, such as circular, oval,square, rectangular, triangular, diamond, pentagonal, hexagonal,heptagonal, and octagonal geometries as well as combinations comprisingat least one of the foregoing geometries. Structures wherein the lengthof the sides are equal (besides a difference caused by the curvature ofthe angle formed by adjacent sides) have been particularly useful inobtaining the desired crush characteristics. In other words,substantially square cells having rounded or 90 degree corners have beenparticularly useful.

The material of the rail extension can be any thermoplastic material orcombination of thermoplastic materials that can be formed into thedesired shape and provide the desired properties. Examples of polymersinclude thermoplastic materials as well as combinations of thermoplasticmaterials elastomeric material, and/or thermoset materials. Possiblethermoplastic materials include polybutylene terephthalate (PBT);acrylonitrile-butadiene-styrene (ABS); polycarbonate; polycarbonate/PBTblends; polycarbonate/ABS blends; copolycarbonate-polyesters;acrylic-styrene-acrylonitrile (ASA);acrylonitrile-(ethylene-polypropylene diamine modified)-styrene (AES);phenylene ether resins; blends of polyphenylene ether/polyamide;polyamides; phenylene sulfide resins; polyvinyl chloride PVC; highimpact polystyrene (HIPS); low/high density polyethylene (L/HDPE);polypropylene (PP); expanded polypropylene (EPP); and thermoplasticolefins (TPO). For example, the polymer can comprise Xenoy™ resin,and/or Noryl™ GTX resin, which is commercially available from SABIC. Thepolymer can optionally be reinforced, e.g., with fibers, particles,flakes, as well as combinations comprising at least one of theforegoing. For example, glass fibers, carbon fibers, and combinationscomprising at least one of the foregoing. For example, the plasticinsert can be formed from STAMAX™ materials, a long glass fiberreinforced polypropylene commercially available from SABIC. Theextension can also be made from combinations comprising at least one ofany of the above-described materials and/or reinforcements, e.g., acombination with a thermoset material.

The overall size, e.g., the specific dimensions of the rail extensionwill depend upon the particular vehicle, the desired crushcharacteristics, and the space available. For example, the length (l),height (h), and width (w) of the rail extension, will depend upon theamount of space available between the rail and the bumper beam of thevehicle as well as crush characteristics (e.g., desired displacement).(See FIG. 1) The design of the cells, the angle and existence ofchamfered section, and the thickness of the cell walls will depend uponthe desired crush characteristics (e.g., maximum force exerted by therail extension during an impact (e.g., while crushing)). The length, l,of the rail extension can be less than or equal to 300 mm, specifically,50 mm to 250 mm, and more specifically 100 mm to 200 mm (e.g., 150 mm)The width, w, of the energy absorbing device can be less than or equalto 200 mm, specifically, 20 mm to 150 mm, and more specifically 40 mm to100 mm. The height, h, of the energy absorbing device can be less thanor equal to 300 mm, specifically, 60 mm to 200 mm, and more specifically80 mm to 150 mm. The length is greater than or equal to the height whichis greater than or equal to the width. The length, height, and widthmeasurements are the broadest measurement in the specified direction,excluding vehicle attachment tabs. The thickness of the cell walls canbe up to 5.0 mm, specifically, 2.0 mm to 4.5 mm, and more specifically3.0 mm to 4.0 mm.

As with the dimensions of the components, the number of cells isdependent upon the desired stiffness, crush characteristics, andmaterials employed. For example, the rail extension can have up to 50cells or more, specifically, 5 to 25 cells, more specifically, 8 to 15cells.

The rail extensions disclosed herein are configured to absorb asignificant amount of impact energy when subjected to axial loadingwhile also having acceptable creep performance (i.e., less deformationupon impact). For example, the rail extension can have a creepperformance when subjected to 4.5 megaPascals (MPa) stress loading for600 hours at 90° C. of negligible deformation (less than or equal to 5mm, specifically, less than or equal to 3 mm, and more specifically,less than or equal to 1 mm).

The rail extensions can be produced by various molding processes, withinjection molding generally employed in order to get the desired wallthickness consistency.

A more complete understanding of the components, processes, andapparatuses disclosed herein can be obtained by reference to theaccompanying drawings. These figures (also referred to herein as “FIG.”)are merely schematic representations based on convenience and the easeof demonstrating the present disclosure, and are, therefore, notintended to indicate relative size and dimensions of the devices orcomponents thereof and/or to define or limit the scope of the exemplaryembodiments. Although specific terms are used in the followingdescription for the sake of clarity, these terms are intended to referonly to the particular structure of the embodiments selected forillustration in the drawings, and are not intended to define or limitthe scope of the disclosure. In the drawings and the followingdescription below, it is to be understood that like numeric designationsrefer to components of like function.

FIG. 1 is a partial prospective view of a part of a body-in-white 32,wherein rail extensions 4 are connected to the vehicle rails 6 and tothe bumper beam 2. On an opposite side of the bumper beam 2 can be crashcans 6, an energy absorber 34 comprising energy absorbing crush lobes36, and a fascia 38. Desirably, the system (e.g., the rails, railextensions, and bumper beam) have a natural frequency

The rail extensions can furthermore have a natural frequency that ismore than the frequency of vibration loading (excitation), specifically,the natural frequency is more than or equal to 70% of the vibrationloading, and more specifically, the natural frequency is more than orequal to 40% of the frequency of vibration loading.

FIGS. 2-4 illustrate examples of rail extensions with different optionalvehicle attachment tab locations. In these figures, it can be seen thatthe rail extensions can have channels 16 along two or more sidesthereof. The channels 16 allow complete crushing of the extensionsduring an impact at an impact force of greater than or equal to 60 kN,specifically greater than or equal to 80 kN, more specifically greaterthan or equal to 90 kN, with the upper limit being the force limit ofthe rails. In other words, the middle cell for two, specifically allfour, sides of the rail extension, has only 3 sides, forming an open, 3sided channel. During crushing, the polymer of the cells is forced intothe channel(s), allowing the extension to crush completely. For example,the walls start deforming and start folding in wavy manner to absorbenergy during loading, once all walls are folded completely resulting instack up (−25% of complete length of rail extension) that cannot becrushed more and load start getting transfer to rails. In other words,the extension can comprise opposing “U” shaped cell groups with cell(s)joining the bases of the “U”. (See FIG. 2, design illustration 40. Thisparticular design (e.g., combination of reverse “U”) allows the walls tofold in systematic manner (e.g., a wavy pattern) to attain a completecrushing, and absorb energy during deformation. A symmetric design, inwhich all walls are closed, could be very stiff and not allow completecrushing.

Further tuning of the crush characteristics can be attained by bevelingthe outermost cells of the extension on two opposing sides to formchamfered cells 14. The beveling can be at an angle from the bumper sideface 18, of greater than 0° to 45°, specifically, 10° to 40°, morespecifically, 15° to 35°. The number of chamfered cells 14 is dependentupon the maximum force that can be exerted during a crash. Bevelingcell(s) reduces the number of cells in physical contact with the bumper(see FIG. 1), and therefore reduces the initial force attained during animpact. The specific angle desired for a particular design can bedetermined by measuring the force transferred to the rails versusdisplacement upon a frontal crash, to determine if the maximum force isexceeded. As the angle increases, the initial force peak decreases.

As is illustrated in FIGS. 2-4, the rail extensions have vehicleattachments 12 that extend from the ends thereof. The vehicleattachments 12 extending from the front face 18 can be employed toattach the rail extension to the bumper beam. The vehicle attachments 12extending from the rear face 20 (rail side face) can be employed toattach the rail extension to the vehicle rails. Optionally, the vehicleattachments (e.g., tabs that extend perpendicular to the length of therail extension), can have reinforcement inserts (attachment inserts)8,10. The reinforcement inserts 8,10 reduce the stress on the railextension due to the attachment to the vehicle. Optionally, theattachment inserts can be metal or a different polymer than the cells.For example the reinforcement inserts 8,10 can be overmolded in the tabsso as to form integral elements of the tabs. These inserts can be metalwith a hole therethrough configured to receive a bolt or the like forattachment to the vehicle.

FIG. 5 is a graphic illustration of displacement versus force for asimulation and for a test sample. The design of the test and thesimulations were as illustrated in FIG. 2, absent the vehicle attachmenttabs. The cell thickness was 3.6 mm, the chamfer angle θ was 30°, andthe material was Xenoy™ resin. As can be seen, for the test sample, thetest sample maintains a force close to the maximum force (100 kN),without substantially exceeding the maximum force (also referred to asthe nominal force). In other words, the force during the crushing, to acomplete crushing of 100 mm, is 80 kN to 105 kN. Furthermore, the peakforce does not exceed 105 kN; i.e., when the initial force is applied,and the rail extension begins to crush (e.g., displacement of less thanor equal to 20 mm, the force does not exceed 105 kN).

FIG. 6 is a graphic illustration of displacement versus force for asimulation of the present plastic rail extension (from FIG. 5) and for ametal rail extension. As can be seen, for the test sample, the testsample maintains a force close to the maximum force (100 kN), withoutsubstantially exceeding the maximum force, especially during the initialpeak. In contrast, the metal rail extension significantly exceed themaximum force during the initial peak. Such crush characteristics canresult in damage to or failure of the rails.

It is noted that 100 kN is provided as the maximum force for theseparticular examples, however, the actual maximum force can be differentand is dependent upon the maximum force that can be applied to the railbefore it begins to crush. The force peak of the rail extension and thevariability of the force during displacement can be tuned (e.g.,adjusted), by adjusting the thickness of the cells, the design of thecells, and the bevel of the outermost cells in contact with the bumperbeam.

Desirably, the rail extension has a force during crushing of 0.60x to1.05x, specifically, 0.75x to 1.03x, and more specifically, 0.80x to x,wherein “x” is the maximum force of the rail before it will begincrushing.

Set forth below are some embodiments of the rail extension, vehiclescomprising the rail extensions, and methods of making the railextension.

Embodiment 1

a rail extension, comprises: an energy absorber comprising a polymerbody, and vehicle attachment tabs extending from one end of the energyabsorber and configured to attach to a vehicle rail, and an attachmenttab extending from another end of the energy absorber and configured toattach to a bumper beam. The energy absorber can comprise cells formedby cell walls extending a length of the energy absorber and formingcavities therethrough; and open channels formed on each side of theenergy absorber, wherein the channels are defined by walls of adjacentcells.

Embodiment 2

The rail extension of Embodiment 1, wherein the energy absorber consistsof a polymer body.

Embodiment 3

The rail extension of any of Embodiments 1-2, wherein cells adjacent tochannels on two opposing sides of the energy absorber have a beveledsurface.

Embodiment 4

The rail extension of Embodiment 3, wherein the beveled surface has achamfer angle θ of greater than 0 to 45°.

Embodiment 5

The rail extension of Embodiment 4, wherein the chamfer angle θ isgreater than 15° to 35°.

Embodiment 6

The rail extension of any of Embodiments 1-5, wherein the energyabsorber has a width and a height, and wherein the length of less thanor equal to 300 mm, the width, w, of less than or equal to 200 mm, andthe height, h, of less than or equal to 300 mm, wherein the length isgreater than or equal to the height which is greater than or equal tothe width.

Embodiment 7

The rail extension of Embodiment 6, wherein the length, l, is 50 mm to250 mm, the width, w, is 20 mm to 150 mm, and the height, h, is 60 mm to200 mm.

Embodiment 8

The rail extension of any of Embodiments 1-7, wherein a thickness of thecell walls is up to 5.0 mm.

Embodiment 9

The rail extension of Embodiment 8, wherein the thickness of the cellwalls is 2.0 mm to 4.5 mm.

Embodiment 10

The rail extension of any of Embodiments 1-9, wherein the rail extensionhas a force during crushing of 0.60x to 1.05x, wherein “x” is themaximum force of the vehicle rail before it will begin crushing.

Embodiment 11

The rail extension of Embodiment 10, wherein the force during crushingis 0.75x to 1.03x.

Embodiment 12

The rail extension of any of Embodiments 1-11, wherein the cavitiescomprise foam.

Embodiment 13

The rail extension of any of Embodiments 1-12, wherein, during animpact, the rail extensions begin crushing upon an impact of greaterthan or equal to 60 kN.

Embodiment 14

The rail extension of any of Embodiments 1-13, wherein, during animpact, the rail extensions have a first peak force of less than orequal to 110% of a force limit of a rail to which the rail extension isattached.

Embodiment 15

A vehicle can comprise: a bumper beam; a vehicle rail; the railextension of any of Embodiments 1-14.

Embodiment 16

The vehicle of Embodiment 15, further comprising a vehicle energyabsorber comprising crush lobes, wherein the bumper beam is between thevehicle energy absorber and the rail extension; and a fascia, whereinthe energy absorber is located between the bumper beam and the fascia.

Embodiment 17

The vehicle of any of Embodiments 15-16, wherein an system comprisingthe bumper beam, rail extension, and rail, has a normal loadingfrequency that is less than a natural frequency of the system.

Embodiment 18

The vehicle of Embodiment 17, wherein the normal loading frequency isless than our equal to 75% of the natural frequency.

Embodiment 19

The vehicle of Embodiment 17, wherein the normal loading frequency isless than our equal to 60% of the natural frequency.

Embodiment 20

A method of controlling a crushing of a vehicle extension, can comprise:forming a rail extension, the a rail extension comprising an energyabsorber comprising a polymer body, wherein the energy absorbercomprises cells formed by cell walls extending a length of the energyabsorber and forming cavities therethrough; forming open channels alongthe outside of the rail extension; determining an initial force peakduring crushing of the energy absorber; beveling a surface of some cellsbased upon the initial force peak; changing the angle of the bevel untilthe initial force peak is less than a desired maximum force.

Embodiment 21

The method of Embodiment 20, further comprising filling some of thecavities with foam.

Embodiment 22

The method of Embodiment 20, further comprising filling all of thecavities with foam.

Embodiment 23

The method of any of Embodiments 20-22, wherein the maximum force isless than or equal to 110% of a force limit of a rail to which the railextension will be attached to.

All ranges disclosed herein are inclusive of the endpoints, and theendpoints are independently combinable with each other (e.g., ranges of“up to 25 wt. %, or, more specifically, 5 wt. % to 20 wt. %”, isinclusive of the endpoints and all intermediate values of the ranges of“5 wt. % to 25 wt. %,” etc.). “Combination” is inclusive of blends,mixtures, alloys, reaction products, and the like. Furthermore, theterms “first,” “second,” and the like, herein do not denote any order,quantity, or importance, but rather are used to d one element fromanother. The terms “a” and “an” and “the” herein do not denote alimitation of quantity, and are to be construed to cover both thesingular and the plural, unless otherwise indicated herein or clearlycontradicted by context. The suffix “(s)” as used herein is intended toinclude both the singular and the plural of the term that it modifies,thereby including one or more of that term (e.g., the film(s) includesone or more films). Reference throughout the specification to “oneembodiment”, “another embodiment”, “an embodiment”, and so forth, meansthat a particular element (e.g., feature, structure, and/orcharacteristic) described in connection with the embodiment is includedin at least one embodiment described herein, and may or may not bepresent in other embodiments. In addition, it is to be understood thatthe described elements may be combined in any suitable manner in thevarious embodiments.

While particular embodiments have been described, alternatives,modifications, variations, improvements, and substantial equivalentsthat are or may be presently unforeseen may arise to applicants orothers skilled in the art. Accordingly, the appended claims as filed andas they may be amended are intended to embrace all such alternatives,modifications variations, improvements, and substantial equivalents.

1. A rail extension, comprising an energy absorber comprising a polymerbody, wherein the energy absorber comprises cells formed by cell wallsextending a length of the energy absorber and forming cavitiestherethrough; open channels formed on each side of the energy absorber,wherein the channels are defined by walls of adjacent cells; and vehicleattachment tabs extending from one end of the energy absorber andconfigured to attach to a vehicle rail, and an attachment tab extendingfrom another end of the energy absorber and configured to attach to abumper beam.
 2. The rail extension of claim 1, wherein the energyabsorber consists of a polymer body.
 3. The rail extension of claim 1,wherein cells adjacent to channels on two opposing sides of the energyabsorber have a beveled surface.
 4. The rail extension of claim 3,wherein the beveled surface has a chamfer angle θ of greater than 0 to45°.
 5. The rail extension of claim 4, wherein the chamfer angle θ isgreater than 15° to 35°.
 6. The rail extension of claim 1, wherein theenergy absorber has a width and a height, and wherein the length of lessthan or equal to 300 mm, the width, w, of less than or equal to 200 mm,and the height, h, of less than or equal to 300 mm, wherein the lengthis greater than or equal to the height which is greater than or equal tothe width.
 7. The rail extension of claim 6, wherein the length, l, is50 mm to 250 mm, the width, w, is 20 mm to 150 mm, and the height, h, is60 mm to 200 mm.
 8. The rail extension of claim 1, wherein a thicknessof the cell walls is up to 5.0 mm.
 9. The rail extension of claim 8,wherein the thickness of the cell walls is 2.0 mm to 4.5 mm.
 10. Therail extension of claim 1, wherein the rail extension has a force duringcrushing of 0.60x to 1.05x, wherein “x” is the maximum force of thevehicle rail before it will begin crushing.
 11. The rail extension ofclaim 10, wherein the force during crushing is 0.75x to 1.03x.
 12. Therail extension of claim 1, wherein the cavities comprise foam.
 13. Therail extension of claim 1, wherein, during an impact, the railextensions begin crushing upon an impact of greater than or equal to 60kN.
 14. The rail extension of claim 1, wherein, during an impact, therail extensions have a first peak force of less than or equal to 110% ofa force limit of a rail to which the rail extension is attached.
 15. Therail extension of claim 1 claim 14, wherein the first peak force is lessthan or equal to 105% of the force limit.
 16. A vehicle, comprising, abumper beam; a vehicle rail; a rail extension comprising an energyabsorber comprising a polymer body, wherein the energy absorbercomprises cells formed by cell walls extending a length of the energyabsorber and forming cavities therethrough; open channels formed on eachside of the energy absorber, wherein the channels are defined by wallsof adjacent cells; and vehicle attachment tabs extending from a rail endof the energy absorber and attached to the vehicle rail, and a bumperattachment tab extending from another end of the energy absorber andattached to the bumper beam.
 17. A method of controlling a crushing of avehicle extension, comprising: forming a rail extension, the a railextension comprising an energy absorber comprising a polymer body,wherein the energy absorber comprises cells formed by cell wallsextending a length of the energy absorber and forming cavitiestherethrough; forming open channels along the outside of the railextension; determining an initial force peak during crushing of theenergy absorber; beveling a surface of some cells based upon the initialforce peak; changing the angle of the bevel until the initial force peakis less than a desired maximum force.
 18. The method of claim 17,further comprising filling some of the cavities with foam.
 19. Themethod of claim 17, further comprising filling all of the cavities withfoam.
 20. The rail extension of claim 1, wherein the open channels arethree-sided.
 21. The rail extension of claim 1, wherein, besidesoptional attachment inserts in the tabs, the rail extension consists ofpolymer material.